1. Field of the Invention
This invention relates to an .alpha.-cyanoacrylate-type adhesive composition, and more specifically to an .alpha.-cyanoacrylate-type adhesive composition which has good storage stability and is rapidly curable with a fast setting time.
2. Description of the Prior Art
An .alpha.-cyanoacrylate monomer, a principal component of an .alpha.-cyanoacrylate-type adhesive composition, has unique anionic polymerizability, and begins to polymerize in the presence of a weak anion such as the slight amount of moisture adhering to the surface of an adherend. Consequently, an .alpha.-cyanoacrylate-type adhesive composition can be used to bond almost all materials, except for some inert materials such as polyethylene and Teflon (trademark for polytetrafluoroethylene resins produced by E. I. du Pont de Nemours), firmly within several seconds to several minutes. An .alpha.-cyanoacrylate-type adhesive has therefore gained widespread acceptance as an instantaneous adhesive in industrial, medical and household applications.
The curing of an .alpha.-cyanoacrylate-type adhesive is based on the anionic polymerization of the .alpha.-cyanoacrylate monomer. The anionic polymerization, however, is inhibited when bonding a adherend having an acidic surface, such as a wooden material, a chemically treated surface (e.g., a chromate-treated surface), or a surface which has the tendency to permit the formation of an acidic oxide coating. Frequently, this causes a retardation in the setting time, and the adhesion strength obtained is not necessarily sufficient. For example, wooden materials generally used have a moisture content of about 10% by weight at a temperature of about 23.degree. C. and a relative humidity of about 55%. When a wooden material is bonded with a conventional .alpha.-cyanoacrylate-type adhesive composition, a setting time of several minutes to more than ten minutes is required despite the presence of such a large amount of water in the woody tissue and the surface of the material. It has been generally considered to be difficult to bond wooden materials with .alpha.-cyanoacrylate-type adhesive compositions. The reason is believed to be due to the following. The surface of a wooden material is acidic with a pH of about 4 to 6 because of the sap present, and, therefore, in bonding, the anionic polymerization of the .alpha.-cyanoacrylate monomer is inhibited and the rate of curing is slow. Furthermore, during this time, the adhesive composition penetrates into the porous woody tissue, and adhesive-free portions in the adhesive layer result.
Various investigations have been made to provide a method for shortening the setting time of .alpha.-cyanoacrylate-type adhesive compositions. The most common method is to use a curing agent for promoting the anionic polymerization of the .alpha.-cyanoacrylate monomer as a primer or a post-treating agent (for example, as disclosed in U.S. Pat. No. 3,260,637). When both the .alpha.-cyanoacrylate monomer and the curing agent are prepared as a one-package adhesive composition, the monomer polymerizes during storage. Hence, they must be stored, and the curing agent described above must be used as a primer on the job. Thus, two coating operations are required to apply both the primer and the adhesive, and the coating efficiency decreases drastically.